An R motor generally comprises a rotor with poles which project outwardly and a stator with poles which project inwardly. The rotor is an iron core and comprises a lamination of a number of thin iron sheets. The stator is a ring-shaped iron core and comprises a lamination of a number of ring-formed thin iron sheets and each of the poles carries an electrical coil made of copper. Each pole of the stator acts as an electromagnet and the rotor rotates when the poles of the rotor are drawn by the magnetic fore of the poles of the stator. By monitoring an angle of rotation of the rotor and switching the energisation of the coils C in synchronism with the rotation of the rotor, the rotor is controlled to rotate in the desired direction. An example of an SR motor is disclosed in JP 07 (1995)-274570 A.
The SR motor shown in the publication comprises three phase coils. Each coil has one end connected through a first switching element to a high potential line of a power supply while the other end of the coil is connected through a second switching element to a low potential line of the power supply. The one end of the coil is connected to the low potential line through a first diode which allow a current flow from the latter to the former while the other end of the coil is connected to the high potential line through a second diode which allows a current flow from the former to the latter.
On the basis of information fed from means for detecting an angle of rotation of the rotor, means for detecting a rotation speed of the rotor, means for detecting an actual current passing through each coil and the outside, the target torque is determined for the SR motor and the energisation of the current is controlled as follows: an energisation ON angle, an energisation OFF angle and a target value corresponding to an angle of rotation are determined with respect to each coil on the basis of a target torque and a rotation speed. The first and second switching elements are both turned on when the angle of rotation is reached at the energisation on angle. The first switching element is turned off if a value of an actual current is over the target value of the current while the angle off rotation is not reached at the energisation OFF angle. The first and second switching elements are both turned off when the angle of rotation is reached at the energisation OFF angle.
Any loss should be reduced to improve the efficiency of the SR motor. Loss of the SR motor is in general caused by the following three elements. The first one is a copper loss that is a joule heat dependent on values of electric resistance and energising current. The second one is a mechanical lows that is windage dependent on rotor rotation and friction loss caused at a bearing of the driving shaft connected to the rotor. The third one is an iron loss that is generated since an alternating magnetic field is generated in the iron cores of the rotor and the stator. The iron loss increases in proportion to the rotation speed of the motor and the maximum magnetic density or the square thereof.